The present invention relates to xe2x80x9cactive implantable medical devicesxe2x80x9d as defined by the Jun. 20, 1990 directive 90/385/CEE of the Council of the European Communities, more particularly to pacemaker, xe2x80x9cmultisitexe2x80x9d (triple or quadruple chambers), defibrillator and/or cardiovertor devices, each of whose operation depends upon the detection of cardiac signals spontaneously produced by the heart of the patient bearing the device, to record data, to pose a diagnosis or to apply a suitable therapy, as the case may be.
It is essential that the behavior of the active implantable medical device respects the physiology of the patient or, at the very least, does not create any harm for the patient. One difficulty encountered in this respect lies in the problem of over-detection of the cardiac activity, i.e., situations where the device detects not only a cardiac event itself (e.g., the depolarization wave of the cardiac cavity being considered), but also an artifact associated with that same event, and wrongly considers the artifact as another event which has occurred after the one just detected.
These artifacts can be of various origins. Thus, when the spontaneous cardiac event presents several components of which some occur tardily after other components which produce a detection of the event, for example, the repolarization wave or T-wave follows the depolarization wave or QRS wave complex and can be detected, wrongly, as a new spontaneous vent distinct from the preceding spontaneous event based on detection of the QRS complex.
The same issue exists for the xe2x80x9ccross talk,xe2x80x9d which is a detection in one cavity of an activity, whether a stimulation or a detection event, coming from another cavity, generally, for example, the detection in the atrial cavity of a ventricular event (depolarization or repolarization).
Various techniques have been proposed to mitigate the risks of over-detection. One technique is the application of a filtering function, analog or digital, in order to eliminate the fast components from the repolarization signal. Another technique is the application of refractory periods, for use in the same cavity or between cavities. Yet another technique is the automatic adjustment of the sensitivity of the detection amplifiers, or the automatic adjustment of the gain of these amplifiers.
However, the use of these various techniques is always done to the detriment of a good detection, principally if there is no over-detection. As an example, long refractory periods provide protection of the device against inopportune detections, but they reduce the sensing capacity of the system and are likely to lead to xe2x80x9cfalse negativesxe2x80x9d if they are too long and let an event pass without detecting it. In the same way, in order to ensure the detection of a ventricular fibrillation (xe2x80x9cVFxe2x80x9d), the amplitude of such a signal being low, it is necessary to seek a maximum sensitivity in order to detect those events which should be detected.
There are also risks of making xe2x80x9cfalse positivexe2x80x9d detections, in particular in the case of certain particularly long endocardiac signals. This problem generally concerns patients presenting dilated cardiopathies for which the duration of depolarization, mainly ventricular, is then increased.
The analysis of the time intervals separating detected successive events (e.g., RR intervals in the case of the ventricular events) is realized in various diagnosis algorithms inside pacemakers, as the one which is described, for example, in the EP-A-0 626 182 and its counterpart U.S. Pat. No. 5,462,060 (both commonly assigned herewith to Ela Mxc3xa9dical). Double detections are obviously likely to deceive this type of algorithm and to lead to sub-optimal actions of the pacemaker (here and in the following discussion, one will understand the term xe2x80x9cpacemakerxe2x80x9d to refer to the pulse generator on its own or as a component of a defibrillator or cardiovertor).
Thus, for a patient presenting a normal sinusal heartbeat rate, the presence of double detections would result in a determination of an average rate as being at a level that is much higher than the real rate. In such a case, there is a risk of application of an undesirable antitachycardic therapy (e.g., a cardioversion shock or a defibrillation shock) in the case of a cardiovertor/defibrillator. The application of the foregoing means would have as a consequence a risk of non-diagnosis of the VF.
The known protection systems described above are not adequately capable of protecting the device against a double detection from the same cardiac event.
It is, therefore, an object of the present invention to minimize risk of double detection, by means applicable in particular to the typical situations described above, without reducing the sensing capacity of the device.
The starting point of the invention lies in the observation by the inventors that on the one hand, the doublet wave detected resulting from a xe2x80x9cdouble detectionxe2x80x9d (i.e., the detection of an event and an artifact related to the event), is wrongly considered by the device as a succession of two distinct events, generally takes place prematurely in the cardiac cycle, and on the other hand, the interval of time separating two successive doublets is longer than the one separating the two detections of the doublet. Stated in other words, the event-artifact interval is shorter than the following artifact-event interval.
This situation of an erroneous xe2x80x9cdouble detectionxe2x80x9d differs from the situation of arrhythmia, where the intervals are generally short and stable for tachycardias, and unstable for fibrillations. Those arrhythmia situations are revealed by detections of frequently occurring events, which are not to be confused with double detections.
The guiding principle underlying the invention then is the recognition of the short interval/long interval alternation. This alternation reveals the presence of a double detection. This phenomenon can then be taken into account during the analysis of the detected signal, in particular by the means for analyzing the signals which determines the cardiac rate from successive detections.
Thus, if one designates the letter L to represent a long cycle corresponding to the normal sinusal coupling (i.e., the time by which a ventricular event follows an atrial event under normal conditions), the letter C to represent a short cycle (typically due to an artifact) and the letter M to represent a cycle of intermediate duration intervening after a cycle C, one obtains the following relations in the case of the erroneous detection of a late occurring signal:
duration L≈duration C+duration M
number of C cycles=number of M cycles
a C cycle is always followed by an M cycle (a xe2x80x9cCMxe2x80x9d pattern)
One standard sequence describing a phenomenon of late detection over a number of cycles could be: L L L C M C M C M C M L L.
Broadly, the present invention is directed to an implantable medical device of the known type including: means for detecting spontaneous events in a cardiac cavity, including a ventricular or an atrial cavity; means for measuring the intervals separating the successive events collected by the detection means; means for analyzing the cardiac rate, operating-according to the values of intervals thus measured; and means for eliminating double detections of the same event, when an event is followed by an artifact also likely to be collected by the means of detection.
According to the invention, the elimination means is capable of identifying an alternation of short intervals and long intervals in the successive intervals separating a series of consecutive events collected by the detection means and, in the presence of such a proven and regular alternation, to announce or indicate to the analysis means that there was a double detection. The analysis means can then take steps to eliminate the tardy detection event and thus treat the double detection as a single event for purposes of its operation.
In a preferred embodiment, the elimination means includes means for performing a statistical analysis of the distribution of the values of the aforesaid successive intervals. Such a statistical analysis means can in particular include: means for classifying the successive intervals according to the length of the interval, distributing the intervals in a histogram comprising a plurality of classes corresponding to consecutive ranges of interval values; and means for determining, starting from the number of intervals classified in each range, the presence of two statistically equivalent peaks in the histogram and, in such a case, to consider that there was double detection.
In one preferred implementation, the statistical, analysis means operates to determine that there is a presence of two statistically equivalent peaks only if: (1) the two peaks are distinct, for example, separated by at least one empty class; (2) there is a principal peak which gathers more than some percentage, e.g., 40%, of the total number of the intervals; and (3) there is a secondary peak which gathers about as many, if not the same number of, intervals as the principal peak.
An additional criterion which can be employed to discriminate double detections is to evaluate the duration C of the short cycles, the duration L of the long cycles and the duration M of the intermediate cycles, and to announce to the means for analyzing the heartbeat rate that there was double detection if: L≈C+M.